Fed or Fasted? Should I Be Training on an Empty Stomach?

For a long time, people have been told to train in the morning on an empty stomach (in a fasted state) to increase fat-loss.

So, what does the scientific research say?

A quick review of the scientific literature shows us some interesting things:

Training in a fasted state doesn’t always seem to have an effect on fat vs. carbohydrate use during exercise,(1) even though we see a decrease in exercise-induced glycogen breakdown and an increase in some of the proteins involved in fat handling after fasting training,(2) and fasted training reduces the amount of blood glucose ‘drop’ during training.(3)

Interval training in the fed or fasted state is equally effective for reducing body-fat,(4) and body composition changes associated with aerobic exercise in conjunction with a calorie-restricted diet are similar, regardless of whether someone is fasted or fed prior to training.(5) And for those fasting for relatively long periods (e.g. greater than 12 hours, such as during the Ramadan fast) there is likely to be no difference in bodyfat or muscle changes whether training in a fasted or fed state.(6) Furthermore, there doesn’t appear to be any significant increase in thermogenesis as a result of fasted training, over non-fasted.(7)

So, it seems from that, that the case is closed and there is no benefit to training fasted…

But wait! There’s more…

When people are not eating a calorie restricted diet, the results of fasted training might be quite different. A couple of studies have looked at diets that aren’t calorie restricted. In these diets training in a fasted state controlled bodyweight, enhanced full-body glucose tolerance and improved insulin sensitivity along with some underlying benefits for fat metabolism.(8, 9) A 12-hour overnight fast also increases the use of fat-for-fuel during lower intensity aerobic bouts,(10) and In a fasted state glucose use is lower and fat use is higher.(11) And for those of you who are following lower-carb, high-fat and ketogenic diets, fasted state exercise increases ketogenesis (production of ketones) in nutritional ketosis(12, 13) and so, might help you to get into and stay in ketosis (the unique state of fuel use and fat-burning that comes in a very low carb and high fat diet).

For those following a ‘targeted’ ketogenic approach or doing ‘carb back-loading’ and eating a higher carbohydrate meal after fasted training there appears to be an enhanced glycogen replenishment effect after fasted training.(14) (Although this study used fairly long training sessions of two hours).

Because of the somewhat mixed results in studies, it has been suggested that a mixed-approach is likely to be best and that doing at least some training sessions fasted is beneficial.(15)

But aside from what the research tells us, there are some interesting clinical observations too. I have worked over the years with a lot of emergency responders, special forces soldiers and others that may not always have the luxury of eating to fuel performance. With these people, it has been interesting to see that as they become more used to training, competing and performing in a fasted state, they become much better at it, to the point where many see no real difference between performance whether fed or even in a prolonged fasting situation. So, in this way, training fasted can improve your overall resilience. Think about it this way; if you get up early to go to the gym and you currently feel like you ‘need’ to eat before you train, wouldn’t it be easier to just wake up and go instead?

Based on the evidence there are a few takeaway points that can help you decide whether fasted training is beneficial for you.

There is no need to eat before training

If you are currently dieting (restricting calories heavily) there may not be any additional benefit from fasted training, but it’s also not going to hinder your results

But, if you are not currently dieting, fasted training can help to reduce or eliminate unwanted fat gain

If you are following an LCHF diet, fasted training might help you to become even more fat- and keto-adapted

You can ‘learn’ to perform while fasted and this could save you time and energy!

Give fasted training a try and see if it works for you. Try combining it with some of our HIIT training sessions, especially over the winter when you might be indulging a little more than usual!

Cliff Harvey is a registered clinical nutritionist, strength coach and naturopath. He has been coaching people ranging from world champion athletes through to the chronically and acutely unwell to perform at their best, since the 1990s. He is a former world champion strength athlete and world record holder. Cliff is the founder of the Holistic Performance Institute, a private post-tertiary college, and is a doctoral candidate and researcher at AUT University. He is also the author of six books including his latest release The Carbohydrate Appropriate Diet.